Parchment was the main writing support material in the Middle Ages in Western Europe. It is made from an untanned animal skin, which is preserved by liming, scraping and drying under tension. Parchment is very sensitive to heat and water, which causes in extreme case the denaturation of collagen, its main constituent, to gelatin. The measurement of the shrinkage temperature, by differential scanning calorimetry (DSC) or the micro-hot table (MHT) method, is commonly used in the cultural heritage field to assess the degradation state of collagen-based materials. However, these techniques are invasive, as they require a sample, and destructive, which is an issue in the case of some historical artifacts. The aim of this work is to demonstrate the potential of nonlinear optical (NLO) microscopy to investigate in a non-invasive and quantitative way the conservation state of historical parchments.
NLO microscopy enables three-dimensional (3D) imaging with micrometer-scale resolution based on an intrinsic optical sectioning. A key advantage is its multimodal capability. Two-photon excited fluorescence (2PEF) signals are emitted by a wide range of materials (fluorophores) in historical artifacts with specific absorption and emission fluorescence spectra . SHG signals are specific for dense and well aligned structures such as fibrillar collagen, and vanish for centrosymmetric materials such a gelatin. Accordingly, SHG microscopy provides structural information about the 3D organization of the fibrillar collagen within parchments and other skin-based artefacts [2,3]. Notably, it enables in situ non-invasive assessment of parchment degradation, which is characterized by the loss of the SHG signal and the onset of a 2PEF signal .
In order to quantify intermediate states of degradation, we further implement polarization-resolved SHG (P-SHG) microscopy: the SHG intensity is recorded as a function of the linear polarization orientation of the excitation for each pixel of the image. P-SHG microscopy provides two quantitative information: the main orientation and the degree of orientation disorder at the submicrometer scale (Figure 1). P-SHG images are acquired in a set of modern parchments that were artificially degraded by exposure to dry heat for increasing duration. The degradation state of the collagen in these parchments is assessed using DSC. P-SHG data are in good agreement with DSC measurements and prove to be a complementary investigation tool that requires no sampling. Most importantly, P-SHG is shown to reveal the earliest states of degradation.
At the end the 2nd World War, Chartres’ library was partially destroyed. The manuscripts were exposed to fire and then water. The degradation states of the manuscripts are heterogeneous (Figure 2). P-SHG microscopy was performed on these manuscripts to determine their conservation states. Moreover, these analysis were also performed in restored and unrestored parchments and show that the performed restoration did not alter the conservation state of the fibrillar collagen within the parchment.
All these results show the high potential of NLO microscopy for in situ quantitative measurements of the conservation state of historical parchments. Moreover, this methodology could be extended to other cultural heritage materials.
 G. Latour et al., Opt. Express 20, 24623 (2012)
 G. Latour et al., Sci. Rep. 6, 36344 (2016).
 L. Robinet et al., ICOM-CC 18th Triennial Conference Preprints, art. 1609 (2017).